Method and apparatus for emission guided radiation therapy

ABSTRACT

An apparatus comprising a radiation source, coincident positron emission detectors configured to detect coincident positron annihilation emissions originating within a coordinate system, and a controller coupled to the radiation source and the coincident positron emission detectors, the controller configured to identify coincident positron annihilation emission paths intersecting one or more volumes in the coordinate system and align the radiation source along an identified coincident positron annihilation emission path.

RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C 119(e) to U.S.Provisional Patent Application Ser. No. 61/036,709, filed Mar. 14, 2008,the contents of which are incorporated herein by reference in theirentirety.

FIELD OF TECHNOLOGY

This disclosure relates to apparatus and method for radiation therapyand more particularly to apparatus and method for treating cancer tissuein the body using high energy radiation.

BACKGROUND

Radiation therapy (RT) is a method for treating cancerous tissue in thebody using high energy radiation (e.g. x-rays) to kill tumor cells.There are two main types of RT: internal beam and external beam.Internal beam RT is achieved by implanting radioactive material withinthe patient inside or near the cancerous site to be treated. Externalbeam RT is achieved by aiming a high energy beam of radiation throughthe patient so that it passes through the region to be treated. ExternalRT has evolved significantly over the past few decades. In an effort toapply a lethal radiation dose to a tumor while sparing healthy tissue,techniques such as three-dimensional conformal beam RT are used to shapethe beam to match the two-dimensional projection of the tumor onto thepatient surface. Furthermore, the beam is applied at various anglesaround the patient and with varying intensities so as to maximize doseto the tumor while minimizing dose to the surrounding healthy tissue.This is known as intensity-modulated RT (IMRT).

However, uncertainty associated with tumor location and motion can limiteffectiveness of external beam RT. Static errors arise from patientsetup variability as well as natural changes in the tumor location dueto shifting of the internal organs. These can Change between treatments.Dynamic errors arise from tumor motion during treatment (e.g. due topatient breathing). Lung tumors, for example, are known to move on theorder of 1-2 cm during normal patient respiration. This continuingproblem has resulted in a new class of RT systems: image-guided RT(IGRT). These techniques involve imaging the tumor region using aconventional Medical imaging modality (x-ray, CT, MRI, PET, etc.) bothbefore and sometimes simultaneously during treatment so that the tumorlocation can be known at the time of treatment.

IGRT techniques, however, suffer either from a lack of specificity ofthe tumor imaging in many cases it is nearly impossible to visualize thetumor boundaries from x-ray CT), or from poor temporal resolution (PETis the most sensitive modality to imaging cancer however it take minutesto form a good quality PET image). In either case, it is still verydifficult to dynamically track a tumor during RT.

Positron emission tomography (PET) is a medical imaging modality that isfrequently used to detect cancerous tissue in the body. A moleculelabeled with a radioactive atom, known as a PET radiotracer, is firstinjected into the patient. The radioactive atoms inside the patientundergo radioactive decay and emit positrons. Once emitted from an atom,a positron will quickly collide with a nearby electron after which bothwill be annihilated. Two high energy photons (511 keV) are emitted fromthe point of annihilation and travel in opposite directions. When thetwo photons are simultaneously detected by two PET cameras, it is knownthat the annihilation occurred somewhere along the line joining the twoPET cameras. This line is called a positron annihilation emission path.The information collected from thousands of these emission paths is usedto gradually assemble an image of the PET radiotracer distribution inthe body. The most commonly used PET radiotracer is fluorine-18fluorodeoxyglucose (FDG). This is a glucose substitute and therefore isused to image the rate of metabolic activity in the body. Becausecancerous tissue tends to be more metabolically active then healthytissue, there is an increase in FDG uptake in a tumor relative to normaltissue and therefore an increase in the PET signal. FDG-PET is one ofthe most sensitive imaging modalities that can be used to detect thepresence of cancer. It is used extensively for both diagnosis of cancerand monitoring of therapy. However, it is impractical to use PETsimultaneously with external beam RT. PET imaging takes on the order of10 minutes to acquire an image of reasonable quality which severelylimits the use of PET as an agent for dynamic tracking of tumorposition.

SUMMARY

The present subject matter relates to apparatus and method for scanningand aligning radiation along coincident positron annihilation emissionpaths. A method includes detecting a coincident positron annihilationemission path from a radioactive event intersecting a predeterminedvolume during a session, and aligning a radiation source along theemission path during the session. Various examples include repeated,timely alignment of radiation in response to individual detectedemission events. Various examples include receiving location data toidentify predetermined volumes and avoid directing radiation toradiation sensitive areas.

An apparatus is provided for aligning radiation during a radiationsession. The apparatus comprising a radiation source, coincidentpositron emission detectors configured to detect coincident positronannihilation emissions originating within a coordinate system, and acontroller in communication with the radiation source and the coincidentpositron emission detectors, the controller configured to identifycoincident positron annihilation emission paths intersecting one or morevolumes within the coordinate system and to align the radiation sourcealong an identified coincident positron annihilation emission path.

This Summary is an overview of some of the teachings of the presentapplication and is not intended to be an exclusive or exhaustivetreatment of the present subject matter. Further details about thepresent subject matter are found in the detailed description and theappended claims. The scope of the present invention is defined by theappended claims and their legal equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus for aligning radiation along positronannihilation emission paths according to one embodiment of the presentsubject matter.

FIG. 2 is a flowchart for a method of aligning radiation alongcoincident positron annihilation emission paths according to oneembodiment of the present subject matter.

FIG. 3 is a flowchart for a method of aligning and directing aprescribed dose of radiation according to one embodiment of the presentsubject matter.

FIG. 4A-4D illustrates an apparatus for aligning radiation alongpositron annihilation emission paths according to one embodiment of thepresent subject matter.

FIG. 5 shows a collimator assembly according to one embodiment of thepresent subject matter.

FIGS. 6A-6C shows a C-arm gantry for aligning radiation along positronannihilation emission paths according to one embodiment of the presentsubject matter.

DETAILED DESCRIPTION

The following detailed description of the present invention refers tosubject matter in the accompanying drawings which show, by way ofillustration, specific aspects and embodiments in which the presentsubject matter may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice thepresent subject matter. References to “an”, “one”, or “various”embodiments in this disclosure are not necessarily to the sameembodiment, and such references contemplate more than one embodiment.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope is defined only by the appended claims,along with the tall scope of legal equivalents to which such claims areentitled.

The present subject matter relates to a new class of techniques termedemission guided radiation therapy (“EGRT”). One EGRT method includesusing an emission modality that is highly sensitive to cancer directlyduring the treatment stage by responding to individual emission eventsusing a beam of radiation along the detected path of emission. If theradiation response occurs within a sufficiently short time period afterthe emission detection, the tumor site will not have moved substantiallyand will be irradiated. Thus, tumor tracking is inherently achieved inemission guided radiation therapy and complete knowledge of the actualtumor location or motion is not required. By responding to a series ofemission events from the tumor site with a series of respectiveradiation beams, treatment can be achieved effectively irrespective oftumor location uncertainty. It is also possible to treat more than onetumor simultaneously in this fashion. In addition, the same pre-planningprocedure as is employed for current RT protocols may be carried out toidentify a volume within which the tumor will always be present(including its motion) so that no radiation, or minimal radiation, isapplied to regions where the tumor is not present, and/or so thattreatment avoids radiation sensitive organs in the body.

FIG. 1 shows an apparatus for sensing and aligning radiation alongdetected positron annihilation-emission paths according to oneembodiment of the present subject matter. In various embodiments, theapparatus 130 includes a circular moveable gantry supported by a frame(not shown), a radiation source 131, positron annihilation emissionsensors 133, a motion system 134 and a controller 135. In variousembodiments, the apparatus includes x-ray detectors 132 positionedopposite the radiation source to measure radiation applied to a volumeof interest 138. The radiation source 131, x-ray detectors 132 andpositron emission sensors 133 are mounted to the moveable gantry. Thepositron emission sensors 133 are mounted on the gantry between theradiation source 131 and the x-ray detectors 132. In variousembodiments, the apparatus includes x-ray and positron emission sensorscombined so that the positron emission detectors provide sensingcapability around substantially the entire gantry circle from one sideof the radiation source to the other. In one embodiment, the x-raydetectors include, but are not limited to, high energy, mega-electronVolt (MeV) detectors. The positron emission sensors 133 are adapted todetect positron annihilation events by sensing coincident photon paths136 emitted during the events. In various embodiments, the motion systemmoves the gantry and attached equipment about the volume 138 to alignthe radiation source with the path of the detected emission. Thecontroller 135 is connected to the radiation source 131, positronemission sensors 133, x-ray detectors 132 and the motion system 134. Thecontroller 135 identities coincident photon emission paths 136intersecting an identified volume 138 and coordinates the alignment,configuration and triggering of the radiation source 131 to directradiation to the volume 138 along the identified emission paths. Invarious embodiments, the controller receives location information forone or more volumes of interest 138 to limit any applied therapy to aregion of interest. In various embodiments, the controller is programmedwith one or more volumes that the system will not irradiate. Someexamples of these volumes include radiation sensitive areas to protectfrom radiation, or perhaps an area which was treated previously andwhich needs to be untreated in any particular session. In someembodiments, volumes of interest 138 are identified in a first phase ofa radiation session by detecting several positron annihilation emissionsand mapping the activity. In various embodiments, the radiation sourceincludes a collimation assembly 139 for shaping and precisely directingthe radiation from the radiation source.

In various embodiments, the controller moves the radiation source, thepositron emission detectors, or both the radiation source and thepositron emission detectors using a common coordinate system. In oneembodiment, the positron emission detectors are stationary and theradiation source is moveable using a robotic motion system referenced toa common coordinate system. In some embodiments, the radiation sourceand the positron emission detectors are both moveable and use separatemotion systems referenced to a common coordinate system. It isunderstood that various motion systems are possible for moving theradiation source, the positron emission detectors or both the radiationsource and the positron emission detectors without departing from thescope of the present subject matter including, circular gantries,rectilinear gantries, articulated arm robots, custom robotics orcombinations thereof.

FIG. 2 is a flowchart of a method 250 for scanning a volume during aradiation session according to one embodiment of the present subjectmatter. Often, a tumor is imaged a substantial period of time beforeundergoing radiation therapy. During therapy, radiation is directed tothe location the tumor was at when last imaged. Tumors can migratesubstantial distances in relatively short intervals of time. Suchmigrations can result from such innocuous events as a patient changingposture, coughing or even breathing. A typical goal of radiation therapyis to apply enough radiation to the tumor so as to kill the tumor tissuewhile also minimizing radiation applied to non-tumor tissue. If thetumor moves after it has been imaged, some radiation may miss the tumorduring radiation, thus, some portions of the tumor may survive-treatmentand some healthy tissue may receive lethal amounts of radiation.Additionally, if during treatment, radiation is applied to portions ofthe tumor that are dead, the patient will be enduring more radiationthan is necessary. In medical applications, the present method providesreal-time tracking of live tumor tissue and direction of radiation tothe tumor tissue. The method 250 includes detecting coincident positronannihilation emission paths from a radioactive event intersecting avolume 251 and aligning a radiation source along the emission path 252.In various embodiments, the method includes directing radiation alongthe detected emission path to the volume before the content of thevolume moves substantially. In medical applications, applying radiationalong the detected positron annihilation emission path in a timelymanner provides certainty that the radiation therapy is applied toliving tumor tissue; even if the tumor has migrated since last imaged.

FIG. 3 is a flow chart of a method 360 for directing a predetermineddose of external radiation to a volume of interest during a radiationsession according to one embodiment of the present subject matter. Themethod includes receiving location data describing one or more volumesof interest 361. In various situations, the data is generated during apre-treatment planning stage. In some situations, substantial imaginghas been done to diagnose and track a volume of interest such as an areaof diseased tissue. This imaging can be used to establish probablevolumes within which tumors of interest exist and move. The data may begenerated from imaging completed during diagnosis. In some embodiments,location data of more than one volume is received such that radiationmay be directed to several volumes during a session. Identifying volumesof interest, such as cancerous tumor tissue, for example, may include anumber of modalities including, but not limited to, X-Ray imaging,Computed Tomography (CT), Magnetic Resonance Imaging (MRI), PositronEmission Tomography (PET) or combinations thereof. Other modalities arepossible without departing from the scope of the present subject matter.

In various embodiments, prior to a radiation session, a volume ofinterest is provided with a radiotracer. The radiotracer provides asource of positrons for real-time tracking of the volume. The methodthen commences with receiving location data of the volumes 361, if notalready received. In various embodiments, receiving the location data ofthe volume includes registering the volume with a reference point of themachine, such as the radiation source, for example. In variousembodiments, registering the volume with the machine includes performinga low dose CT scan using the machine's radiation source. In someembodiments, receiving location data of one or more volumes andregistering each volume with the machine includes detecting a number ofemission events and mapping the events to identify the volumes. It isunderstood that other methods of registering a volume with the externalradiation machine are possible without departing from the scope of thepresent subject matter. In some embodiments, receiving location data ofthe volumes includes receiving location data of areas not to irradiate.

The method further includes detecting coincident emission paths from apositron annihilation event intersecting a volume of interest 362,aligning the radiation source along the detected emission path 363 andthen directing radiation along the detected emission path to the volume364. In various embodiments, upon detection of an emission event, thedownloaded location data are used to determine whether the eventintersected a volume of interest.

In various embodiments, the radiation source and emission detectors moveabout the one or more volumes to allow for detection of emission eventsand direction of radiation from multiple angles. Directing radiation toa volume from multiple angles minimizes exposure of interveningmaterial, such as healthy tissue, to radiation. In various embodiments,prior imaging is used to regulate intensity of the radiation source toaccount for the depth of the volume within other material. In variousembodiments, as the session progresses, the controller analyzes andconstructs a map of one or more volumes of interest from the emissionevents detected during the session. In various embodiments, as the mapbecomes more detailed, the controller selectively aligns and directsradiation along detected emission paths based on the map.

Upon directing radiation toward a volume, a controller records theamount or dose of radiation directed to the patient 365. In variousembodiments, x-ray detectors located opposite the radiation sourcerecord the radiation directed toward and passing through the volume. Thecontroller monitors the accumulated radiation and continues to detectemission events and direct radiation along detected emission paths untila prescribed dosage of radiation has been directed to each volume ofinterest 366. In various embodiments, accumulation of the detectedpositron emission events can be used to image the volume of interest andsurrounding material. In medical applications, accumulation of thedetected positron emission events can be used to construct an image ofmetabolic activity of the volume. In some embodiments, the image datamay be used to modify subsequent treatment.

In various embodiments, such as those used in medical applications,volumes of interest, such as cancerous tumor tissue, are provided with aradiotracer as a source of positrons for real time tracking of thevolume. A typical radiotracer includes unstable nuclides which emitpositrons. The positron has the same mass as an orbital electron but ispositively charged. A unique characteristic of the positron is that itcan not exist at rest in nature. Once it loses its kinetic energy, thepositron immediately combines with a negatively charged electron andundergoes an annihilation reaction in which the masses of the twoparticles are completely converted into energy in the form of two0.511-MeV annihilation photons, which leave their production site atapproximately 180 degrees from each other. The detection of the two511-keV gamma rays forms the basis for targeting living tumor tissuewith radiotracers.

A commonly used radiotracer in clinical practice and the study ofcancers is fluorine-18 fluorodeoxyglucose (FDG), a metabolic PETradiotracer. FDG, a glucose analog, is taken up by high-glucose-usingcells such as brain, kidney, and cancer cells, where phosphorylationprevents the glucose from being released intact. Thus, living diseasedtissue will take up, and concentrate metabolic PET radiotracers moreintensely than healthy tissue. Because dead tissue does not take up theradiotracer, an added benefit of a metabolic radiotracer is that itprovides read-time tracking of the living tissue of the tumor. As aresult, in applying the radiation therapy along detected emission paths,the method provides a high degree of certainty that the radiation isapplied precisely to not only the tumor, but to the living tissue of thetumor. It is understood that the use of other radiotracers withpositron-emitting radionuclide are possible without departing from thescope of the present subject matter including, but not limited to,Fluorine-18, Carbon-11, Oxygen-15, and Nitrogen-13.

FIGS. 4A-4D show a cross section of an apparatus 401 for detectingcoincident positron annihilation emissions paths and aligning radiationto the emission paths according to one embodiment of the present subjectmatter, FIGS. 4A-4D includes a patient 420 positioned within theapparatus 401. The patient has living tumor tissue within a volume 422to be irradiated. The apparatus includes a controller 407, a radiationsource 402 to produce high energy radiation, a collimation assembly 410to assist shaping and precisely directing the high energy radiation, anarray of x-ray detectors 408; and an array of positron emissiontomography (PET) sensors 404. The radiation source 402; collimationassembly 410, x-ray detectors 408 and PET sensors 404 are situated on arotating gantry 406. The gantry 406 is mounted to a stationary frame(not shown). In various embodiments, a motion control system 409connected to the controller 407 moves the gantry 406 and mountedequipment about the patient 420.

FIG. 4B shows the apparatus 401 performing a low dose, MeV CT scan toregister the patient's position relative to the apparatus 401. Anatomiclandmarks or other marking schemes may be used to register the patient'sbody position with the apparatus. Registration allows the controller torelate the geometric coordinates of the volume(s) of interest, includingthose volumes not to be irradiated, with the geometric coordinates ofthe apparatus 401. After registration, the controller 407 controls theapparatus 401 in monitoring positron annihilation emission pathsintersecting the volume 422, aligning the radiation source to theemission paths and directing radiation along detected emission paths.While monitoring for positron annihilation events, the radiation source402, collimation assembly 410, PET sensors 404 and high energy radiationdetectors 408 rotate around the patient using the motion system 409connected to the controller 407.

FIG. 4C shows detection of a coincident positron emission path 412intersecting the volume 422. Upon detection of a coincident emissionevent and path 412, the controller 407, records the geometriccoordinates of the coincident emission path. If the detected path 412intersects a volume of interest 422, the controller 407 initiatesalignment of the radiation source and triggers radiation along the pathof the emission after the motion system has moved the radiation source402 into alignment with the emission path 412. Applying the radiationalong the same path as the emission provides a high degree of certaintythat the radiation is applied precisely to living tissue of the tumor.In addition to moving the radiation source to the angle of a detectedemission, alignment includes configuring the collimation assembly 410 toprecisely direct the radiation along the positron emission path. Invarious embodiments, precise and timely alignment of the radiationincludes reconfiguring the collimation assembly while the gantry 406 isrotating about the patient.

FIG. 4D shows the apparatus directing radiation along a detectedemission path according to one embodiment of the present subject matter.The controller coordinates the motion of the gantry, configuration ofthe collimation assembly and triggering of the radiation source toprovide radiation 414 from the radiation source 402 along the same pathas detected positron emission paths. The collimation assembly 410 allowsthe radiation beam to be fragmented so that desired rays in the fan-beammay pass through while others are completely occluded. Reconfigurationof the collimation assembly occurs quickly so that a new configurationis possible for each angular position of the radiation source 402 aroundthe gantry 406.

As the gantry 406 rotates, detection, alignment and triggering ofradiation is repeated for multiple positron emission events until adesired radiation dose is achieved in each volume of interest. Invarious embodiments, the controller records readings received from thex-ray detectors 408 to verify the total radiation dosage. It isunderstood that x-ray detectors may be formed of one or more high energydetectors including, hut not limited to, MeV detectors, high energykilo-electron volt (keV) detectors or combinations thereof.

In various embodiments, the controller 407 includes data about thelocation of one or more tumors. In such embodiments, the controllercoordinates detection of emission events and triggering and directing ofradiation to the multiple volumes. In various embodiments, volumelocation data is downloaded into the controller from previous imaging.The data allows the controller to determine with a high degree ofcertainty whether a detected emission event path intersects with avolume of interest.

As discussed above, when positron emission events intersecting a volumeof interest are detected, the controller 407 reconfigures thecollimation assembly 410 to allow radiation from the radiation source402 to follow the same path as the detected emission path within aspecified time. In various embodiments, reconfiguration of thecollimation assembly is accomplished on-the-fly as the gantry 406rotates the radiation source 402 and sensors 404, 408 about the patient420. Speed of the gantry rotation, processing delay of the controller,and location of the radiation source upon detection of an emission eventare some factors that determine the delay between the emission event anddirecting and triggering radiation along the emission path. In medicalapplications, alignment of the radiation source to emission pathsincreases tracking accuracy of tumors even for motion of the tumorsresulting from normal body functions such as breathing. In someembodiments, the gantry includes multiple radiation sources. Multipleradiation sources allow treatment to be completed more quickly, allowdelay reduction between detection of an emission event and alignment ofa radiation source or both quicker treatment and reduced delay.

In various embodiments, the apparatus aligns and directs radiation inresponse to individual positron annihilation events. In variousembodiments, the controller queues detected positron annihilationemissions as the gantry rotates, aligns and directs radiation topreviously detected emission paths. In some embodiments, the radiationsource is aligned and directed sequentially along selected queued paths.In some embodiments, the radiation source is aligned and directed alongselected queued paths depending on the present position of the radiationsource about the volume, such that a more recently detected path may beselected because the radiation source will approach that path before anearlier detected path. In some embodiments, the controller queuesselected emission paths for an interval of time and then aligns anddirects radiation along selected paths of the queue before repeating theprocess by queuing additional emission paths. By adjusting the desiredtime interval between sensing an event and delivering radiation alongthe detected emission path, the apparatus can establish a queue ofdifferent numbers of radiation delivery paths. It can also usealgorithms to provide the radiation with the least amount of movement ofthe apparatus. Other algorithms and procedures are possible withoutdeparting from the scope of the present subject matter.

In some embodiments, the controller paces alignment and triggering ofthe radiation source to cyclical functions of the patient such asbreathing. For example, assume the breathing cycle of a patient has beensensed to repeat every few seconds. As the controller senses emissionevents intersecting a volume of interest, the controller records thephases of the breathing cycle in which these events occurred andcontrols a coordinated delay in moving, aligning and triggering theradiation source to coincide with the same phases of the breathingcycle.

Various embodiments of the present subject matter align and directradiation in a 2-D or a 3-D mode. In a 2-D mode, a multi-leaf radiationcollimator is reconfigured for each specified angular position of theradiation source so that radiation paths lie within a fan whose vertexis at the source of radiation. FIG. 5 shows a collimator assembly 550according to one embodiment of the present subject matter. A pair ofcollimator jaws 551 immediately preceding the multi-leaf collimator 553restricts the conical divergence 554 of the radiation source 555 to aparticular plane 552. Operation of the lower jaws 553 allows alignmentof the radiation to multiple volumes within the plane of the radiationpath. In various embodiments, a table may be translated in a continuousfashion or in a step-and-shoot fashion to treat multiple slices of avolume. If the table is translated continuously, the table speed shouldallow for a prescribed dose to each volume.

In the case of an apparatus employing a 3-D mode of treatment, thecollimator jaws 551 restricting the conical radiation beam divergence554 may be moved in coordination with the collimation assembly leaves553. Although radiation is restricted to a particular plane, coordinatedmotion of the collimation assembly jaws allow various planes oftreatment for a given position of the radiation source. For circulargantries and C-Arm apparatus; 3-D mode allows the collimation assemblyto provide radiation at angles oblique to the central axis of theapparatus. Additionally, for a given position of the apparatus, the 3-Dmode allows the controller to respond to multiple coincident emissionpaths within a larger portion of the field of view of a PET sensor arraycompared to 2-D mode.

FIGS. 6A-6C shows a &arm gantry apparatus 660 according to oneembodiment of the present subject matter. The apparatus 660 includes aradiation source 661, collimation assembly 662 and PET cameras 663mounted to a rotary section 664 of the apparatus. A fixed section 665 ofthe apparatus provides support for the rotary section 664. FIG. 6A showsa front view of the apparatus with the radiation source located above, atable 666 and the PET cameras 663 positioned at either side of the table666. A controller (not shown) provides control and coordination of theradiation source 661, collimation assembly 662, PET cameras 663, andmotion as the rotating section 664 moves the devices about the table666. FIG. 6B shows a front view of the apparatus with the radiationsource 661, collimation assembly 662 and PET cameras 663 rotated 90degrees about the table 666. For 3D mode treatment, a multi-leaf x-raycollimator is reconfigured for each specified angular position of theradiation source so that the radiation paths lie within a cone whosevertex is at the source of radiation.

In various embodiments, in order to treat multiple slices of a volume,the table 666 may by translated in a continuous fashion or in a step andshoot mode. FIG. 6C shows a side view of the C-arm gantry apparatus 660with the table 666 translated toward the apparatus. In variousembodiments, where the table is translated continuously, table speed iscontrolled to allow for a prescribed dose of radiation to be directed toeach volume.

In various embodiments, the apparatus 660 includes high energy (MeV)detectors located opposite the radiation source to record and verify theamount, or dose, of radiation directed to each volume. In someembodiments, instead of MeV detectors, combined MeV/PET detectors areused. Combined MeV/PET detectors allow detection of both 511 keV PETemissions as well as high energy radiation. Such an arrangementincreases the coverage of the PET detectors and allows for a fasterradiation session. It is understood that other combinations of PET andhigh energy x-ray detectors are possible without departing from thescope of the present subject matter including but not limited to keV/PETdetectors.

In various embodiments, the radiation includes x-rays produced by alinear accelerator (linac). Other radiation types and radiation sourcesare possible for providing radiation without departing from the scope ofthe present subject matter. Such radiation and radiation sourcesinclude, but are not limited to, high energy photons, radiation orparticles produced by a radioactive isotope (e.g. iridium or cobalt60),high energy electrons, a proton beam, a neutron beam and a heavy ionbeam.

In one embodiment, the apparatus is implemented using a single photonemission computed tomography (SPECT) setup with pinhole cameras and/orcollimated SPECT sensors in place of the PET sensors to detect thedirection of emitted photons from a radioactive event.

In one-embodiment, the radiation source does not rotate. Radiationsources (e.g. lead encased cobalt60) are placed all around the gantry,alternating with the PET detectors. In this case, radiation from aparticular radiation source would follow the emission path detected byan adjacent PET sensor pair.

In one embodiment, two or more radiation sources are affixed to therotating gantry. The controller aligns each of the radiation sources torespond to distinct emission paths. Multiple radiation sources permit asmaller time window between detecting an emission path and directingradiation along that path, than if only a single radiation source isused.

In various embodiments, the apparatus provides other modes of radiationtherapy for stand alone operation, or for simultaneous operation withemission guided radiation therapy. Other modes of radiation therapyinclude, but are not limited to, radiation treatment based on priorimaging of the treatment volumes, three-dimensional conformal beam RT,intensity-modulated RT or combinations thereof.

This application is intended to cover adaptations and variations of thepresent subject matter. It is to be understood that the abovedescription is intended to be illustrative, and not restrictive. Thescope of the present subject matter should be determined with referenceto the appended claims, along with the full scope of legal equivalentsto which the claims are entitled.

1-25. (canceled)
 26. A radiation therapy system comprising: a gantryrotatable about a tumor region; a radiation source mounted on thegantry; a collimation assembly mounted on the gantry and disposed in aradiation path of the radiation source, wherein the configuration of thecollimation assembly is adjustable while the gantry is rotating; anarray of positron annihilation emission detectors mounted along at leasta circumferential length of the gantry and configured to detect positronannihilation emission paths from the tumor region; and a controller incommunication with the array of positron annihilation emission detectorsand configured to construct a map from the detected positronannihilation emission paths, and wherein the controller is incommunication with the radiation source and configured to directradiation from the radiation source to the tumor region based on themap.
 27. The system of claim 26, wherein the controller is incommunication with the gantry and is further configured to directradiation while rotating the gantry and detecting positron annihilationemission paths.
 28. The system of claim 27, wherein the array ofpositron annihilation emission detectors is configured to detectpositron annihilation emission paths from multiple tumor regions, andwherein the controller is configured to direct radiation from theradiation source to the multiple tumor regions from multiple gantrypositions.
 29. The system of claim 26, wherein the controller isconfigured to direct radiation to the tumor region from multiple gantrypositions.
 30. The system of claim 26, wherein the controller is furtherconfigured to construct an image of metabolic activity of the tumorregion based on the map of positron annihilation emission paths.
 31. Thesystem of claim 26, further comprising a motion system in communicationwith the controller, wherein the controller controls gantry rotationusing the motion system.
 32. The system of claim 26, wherein thecontroller is further configured to record a dose of radiation directedto the tumor region.
 33. The system of claim 32, further comprising anMeV detector mounted on the gantry opposite the radiation source, andwherein the controller is in communication with the MeV detector and isconfigured to record the radiation dose based on MeV detector readings.34. The system of claim 26, wherein the controller is in communicationwith the collimation assembly and the collimation assembly comprises amulti-leaf collimator and collimator jaws.
 35. The system of claim 34,wherein the multi-leaf collimator is reconfigurable for each gantryposition.
 36. The system of claim 35, wherein the controller is furtherconfigured to move the collimator jaws in coordination with themulti-leaf collimator.
 37. The system of claim 26, wherein thecontroller is in communication with the collimation assembly and isconfigured to change a configuration of the collimation assembly todirect radiation from the radiation source along the detected positronannihilation emission paths.
 38. The system of claim 26, wherein thecontroller is in communication with the gantry and the collimationassembly, and wherein the controller is configured to coordinate gantryrotation, collimation assembly configuration adjustment, and radiationsource triggering to direct radiation to the tumor region along thedetected positron annihilation emission paths.
 39. The system of claim26, further comprising a plurality of radiation sources mounted on thegantry.
 40. The system of claim 26, wherein the tumor region comprisesliving tumor tissue.
 41. The system of claim 26, further comprising atable, wherein the gantry is rotatable about the table, and wherein thecontroller is configured to translate the table in steps and isconfigured to direct radiation to the tumor region at each table step.42. A method for radiation therapy comprising: identifying a volume ofinterest by detecting a plurality of positron annihilation emissionpaths from the volume of interest using an array of positronannihilation emission detectors mounted on a rotatable gantry;constructing a map of positron annihilation emission path activity fromthe detected positron annihilation emission paths; and directingradiation from a radiation source mounted on the rotatable gantry to thevolume of interest based on the map of positron annihilation emissionpath activity, wherein directing radiation comprises adjusting aconfiguration of a collimation assembly that is disposed in a radiationpath of the radiation source while rotating the gantry.
 43. The methodof claim 42, wherein directing radiation from the radiation source isbased on a map of positron annihilation emission path activity from aprevious radiation therapy session.
 44. The method of claim 42, furthercomprising constructing an image of metabolic activity of the volume ofinterest based on the map of positron annihilation emission pathactivity.
 45. The method of claim 44, wherein directing radiation fromthe radiation source is based on an image of metabolic activity from aprevious treatment session.
 46. The method of claim 42, wherein thevolume of interest comprises cancer tissue.
 47. The method of claim 42,wherein the volume of interest comprises brain tissue.
 48. The method ofclaim 42, wherein the volume of interest comprises kidney tissue. 49.The method of claim 42, wherein the volume of interest comprises lungtumor tissue.
 50. The method of claim 42, wherein the volume of interestcomprises living tumor tissue.
 51. The method of claim 42, furthercomprising providing a radiotracer to the volume of interest.
 52. Themethod of claim 51, wherein the radiotracer is a PET radiotracer. 53.The method of claim 51, wherein the radiotracer is a metabolicradiotracer.
 54. The method of claim 51, wherein the radiotracercomprises a positron-emitting radionuclide selected from the groupconsisting of fluorine-18, carbon-11, oxygen-15, and nitrogen-13. 55.The method of claim 42, wherein the volume of interest is located on atable and the method further comprises translating the table in stepsand directing radiation to the volume of interest at each table step.